Indirect ophthalmoscopy techniques are now in widespread use in diagnostic and therapeutic procedures in the field of ophthalmology. Indirect ophthalmoscopy techniques include the use of a hand-held lens, in conjunction with a binocular indirect ophthalmoscope, and more recently in biomicroscopic examination or treatment of the fundus using a slit lamp biomicroscope or operating microscope. The so-called hand-held condensing lens used in indirect ophthalmoscopy performs two functions: condensing the light from the source toward the entrance pupil of the eye, thereby illuminating the fundus, and forming an inverted real aerial image of the fundus at approximately the from focal distance of the lens. It has been found that indirect ophthalmoscopy is superior to direct ophthalmoscopy in the examination of retinopathies, retinal separation, retinal tumors, intraocular foreign bodies, and further in the ability to see fundus lesions which may not be viewable if there are opacities of the ocular media. The hand-held lenses used in indirect ophthalmoscopy have been of a variety of types, with each affording some advantages in the examination of the fundus.
The first hand-held indirect ophthalmoscopy lens which was used as a condensing and image-forming lens, included convex spherical surfaces and was of low power. The aerial image produced with such a spherical lens is magnified and inverted, but was considerably blurred, particularly toward the periphery of the formed image. A two element Ramsden style indirect ophthalmoscopy lens was thereafter produced by Rodenstock. This lens design incorporates spherical convex surfaces, and is capable of improved optical quality over that of a single element plano-convex or bi-convex lens, but the limited benefits of such a design are outweighed by increased surface reflections and light loss. Subsequently, improvements were noted by the use of slightly higher powered single element lenses, each having one aspheric surface with the other surface being plano or spherical. Although the use of a single aspherical surface in the indirect ophthalmoscopy lenses does show great improvement over spherical indirect ophthalmoscopy lenses, lens aberrations may remain such that light from the illumination source is not converged to a focus at the entrance pupil of the eye, and the formed aerial image of the fundus may show aberrations and increasing astigmatic effects particularly in the peripheral regions. These designs have been subsequently improved upon with the use of two aspherical surfaces incorporated into the indirect ophthalmoscopy lens. The first use of a double aspheric indirect ophthalmoscopy lens, designed for use with the indirect ophthalmoscope was described in U.S. Pat. No. 4,738,521, by David Volk, wherein a lens for use in indirect ophthalmoscopy incorporates both the front and back surfaces of the lens being aspheric surfaces of revolution of conoid type. This double aspheric lens design substantially improved the quality of the formed aerial image by reducing aberrations including field curvature, astigmatism, and distortion. The use of double aspheric lenses, wherein the surfaces are particularly conoid surfaces, has been found to be of distinct advantage in indirect ophthalmoscopy and has made possible the use of much stronger lenses while providing increased clarity of the image with increased size of the field of view.
In the Jun. 1982 edition of Ophthalmology Times, there was reported the use of a "periscopic lens" for use in slit lamp fundoscopy. This double plano-convex lens system was similar to the Ramsden style indirect ophthalmoscopy lens produced by Rodenstock but of smaller diameter and higher power. In that spherical surfaces were utilized, only limited improvement in optical quality could be realized, with the remaining inherent problems of reduced field as well as reflection and light loss as seen in the Rodenstock lens.
More recently, there has been developed a symmetrical double aspheric indirect ophthalmoscopy lens particularly suited for use with the slit lamp biomicroscope. This lens is described in U.S. Pat. No. 4,627,694, also by David Volk. The symmetrical non-conoid double aspheric lenses as shown in this patent are of small diameter, with the aspheric surfaces described as having decreasing curvature from the apices of the surfaces peripheralward, providing improved correction of aberrations including field curvature, astigmatism, and distortion. Lenses made according to this design have demonstrated themselves to be better suited for use with the slit lamp biomicroscope and have yielded significant improvement in the examiner's ability to see details in the aerial image of the fundus, yet this lens design, like the periscopic lens and its predecessor the El Bayadi lens, does not account for pupil aberrations, which may be inherent in the lens design and which degrade the optical and performance characteristics of a lens, especially as it relates to observation of the fundus image using the slit lamp biomicroscope. Similarly, other prior indirect ophthalmoscopy lenses have apparently neglected completely the effects of pupil aberration in their design.
Particular problems arise when attempting to use a slit lamp biomicroscope for viewing of the aerial image formed by an indirect ophthalmoscopy lens. If the lens is of lower power, the beam of light from the slit lamp light source associated with the biomicroscope cannot be enlarged sufficiently to fill the full aperture of the lens, leaving a considerable portion of the lens unused in its condensing function. Additionally, the longer focal length, lower power lenses pose the problem of requiring positioning of the biomicroscope at a location which exceeds the range of travel built into the instrument. With the development of the double aspheric indirect ophthalmoscopy lenses described above, these problems were overcome by enabling the use of higher powered lenses, allowing greater illumination of the fundus, increased field of view, and a shorter working distance well within the range of adjustment of the slit lamp biomicroscope. Although such lens design improvements have played an important role in present day eye fundus diagnostic and therapeutic techniques, especially with respect to diagnosis of diseases of the vitreous and retina, there has not been developed an indirect ophthalmoscopy lens particularly designed for use with a slit lamp biomicroscope, which optimally corrects for pupil aberrations as well as the more commonly considered aberrations of field curvature, astigmatism, and distortion.
An indirect ophthalmoscopy lens for use with a slit lamp biomicroscope must also be positioned relative to the patient's eye, such that the conjugate focus of the slit lamp light source through the lens is at approximately the center of the entrance pupil of the patient's eye. The lens must thus be positioned a sufficient distance from the entrance pupil to form the conjugate focus of the slit lamp light source at the proper position for greatest illumination of the fundus. For higher powered lenses, the lens is positioned relatively close to the front of the patient's cornea, while the microscope of the slit lamp is positioned at a significant distance from the patient in order to allow observation of the formed aerial image of the fundus. The distance from the aerial image to the biomicroscope apparatus is dependent upon the attributes of the slit lamp microscope and particularly the focal distance of the objective lens system of the microscope. For the purpose of providing a wider field of view of the fundus by means of slit lamp ophthalmoscopy, the particular diameters of the more highly powered prior art lenses have been made relatively large, such that light rays originating at the more peripheral portions of the illuminated fundus, proceeding through the pupil and cornea, are incident upon the posterior lens surface at its periphery. Although refracted through the lens and contributing to the aerial image formation, these peripheral rays, as a result of inadequate lens design, in fact do not provide peripheral fundus imagery to the practitioner viewing through the slit lamp biomicroscope. This is due to the pupil aberrations of the indirect ophthalmoscopy lens, and the fact that the lens design has not addressed the optical characteristics and requirements of the slit lamp biomicroscope itself. It is therefore seen that the field of view and the image quality obtainable by prior art indirect ophthalmoscopy lenses has not been optimized for examination using a slit lamp microscope, the quality of the imagery of the eye pupil as it specifically relates to the slit lamp microscope pupil having been completely neglected. An indirect ophthalmoscopy lens with significant pupil aberration causes excess vignetting of light rays, even at the mid-peripheral portions of the field of view. In certain cases, the rays from the edge of the field of view may completely miss the objective of the slit lamp microscope. While the hand-held indirect ophthalmoscopy lens is positioned in front of the patient's eye such that the focus of the slit lamp illumination is at or :near the center of the pupil of the patient's eye, the lens, remaining in the same location, must also collect rays exiting the patient's eye, and fulfilling its optical functions, provide the maximum obtainable field of view, clear and sharp fundus imagery, and direct the chief rays of such bundles of light to the biomicroscope pupil. Without these aspects of the lens considered in its design, compromised performance will certainly result.